It is well known that heat can be transferred between fluids within a given volume more efficiently as the size of the channels that conduct these fluids decreases and the number of channels increases. At channel dimensions that are considered by most as “microchannel” in size, meaning less than 5 millimeters and more likely less than 2 millimeters in diameter, design problems multiply rapidly. For example, header design (i.e., separation of opposing streams) is one of the most challenging of problems. Especially in the case of countercurrent flow, it is difficult to maintain separation between opposing streams. The usual remedy has been to arrange the opposing streams in cross-flow configuration; i.e., one stream flowing in the X-direction, while the other flows in the Y-direction, so that their respective inlets and outlets are separated. A hybrid of the countercurrent and cross-flow design is the conventional shell-and-tube heat exchanger with baffles including those of multi-pass and serpentine configuration. The problem with these approaches, however, is that heat transfer efficiency suffers either from a poor pattern of temperature difference or flow distribution, or both. The ideal pattern is to arrange the channels in such a way that the hot stream enters from one end while the cold stream enters from the other, along the same axis, such as along the longitudinal axis of the exchanger. This is single-pass countercurrent flow with little or no cross-flow mixing of either stream.
In shell-and-tube heat exchangers, one stream enters the “tube side” via tubes that extend through a vessel that is filled with another fluid flowing generally in the opposite direction. The opposing fluid flows on the so-called “shell-side” of the heat exchanger. Typically, fluid enters the shell at the opposite end from the fluid entering the tube side. Tubes penetrate a “tube sheet” that contains the shell-side fluid within the vessel, thereby preventing shell-side fluid from leaking into headers that distribute the tube-side fluid to and from the tubes. Countercurrent flow is somewhat achieved in this configuration by ensuring that shell-side fluid flows outside the tubes in an opposite direction to the flow of fluid inside the tubes. However, flow is poorly distributed because of the introduction of the fluid via side ports. To improve flow distribution, internal baffles deflect flow along a serpentine rather than straight axial, path. This results in hybrid of counter-current and cross-flow, as previously described. Increasing the number of baffles, increases the degree of countercurrent flow and the associated heat transfer. This results in an increase in pressure drop as the number of baffles increases. However, there is a practical limit to the number of baffles that one can use.
The shell-and-tube configuration has been practiced at a microchannel level using hollow fibers that are typically spun from a polymeric melt. Squeezing the mass of fibers at either end achieves the same effect as a tube sheet. Heat exchangers of this type, however, are limited in both temperature and pressure. For high temperatures or pressure, typically metal channels and metal tube sheets are required. For microchannel heat exchangers the separation of the streams becomes more difficult because of the high density of channels penetrating the tube sheet. The usual remedy is to resort to a parallel-plate configuration in which opposing streams flow through alternating layers. For example, small microchannels can be achieved by etching or stamping a channel pattern into a thin plate and then bonding that plate to another smooth plate. Depending on the depth of the microchannel, the plates can be bonded by either brazing or diffusion bonding. The use of small microchannels improves efficiency in a compact design, but the device may be heavy due to the space taken up by solid, unetched metal underneath the channels.